The leading contender for the European Union’s top job has pledged to work for a “fair deal for Britain” in a boost for David Cameron’s plan to negotiate looser membership terms.

Jean-Claude Juncker, the former prime minister of Luxembourg, said that he would make keeping Britain in the 28-member club one of his top priorities if he became president of the European Commission. His promise will be regarded by Downing Street as a significant concession because Mr Juncker is regarded as an arch-federalist who has previously warned that Britain will not be allowed to rewrite EU rules.